Natural convection heaters, which usually are positioned on a wall (e.g., baseboard heaters), are well known in the art. Typical baseboard heaters of the prior art are shown in FIGS. 1-3. It will be understood that the prior art baseboard heaters as illustrated in FIGS. 1-3 are simplified, for clarity of illustration. (As will be described, the remainder of the drawings illustrate the present invention.)
The flow of air through a prior art baseboard heater 10 is schematically illustrated in FIG. 1. As shown in FIG. 1, the known baseboard heater 10 has several fins 12 for transferring heat to air passing over the fins 12. Typically, the fins 12 are heated by a heating element 14, to which the fins 12 are attached. As is well known in the art, when the air adjacent to the fins 12 is heated due to heat transfer from the fins 12, such air rises. Air at ambient temperature is drawn into the baseboard heater 10 at a lower side thereof accordingly, resulting in circulation of at least a portion of air in the room through the heater 10 due to natural convection.
As schematically illustrated in FIG. 1, when the conventional heater is operating, ambient air from the room (“R”) is pulled into the baseboard heater 10 (arrows 22a, 22b, 22c, 22d) to replace heated air rising upwardly from the heater. The incoming air schematically represented by arrows 22a-22d is drawn generally upwardly into the conventional baseboard heater when it is operating, to form a column 44 of generally upwardly-moving air (FIG. 1). The column of heated air exiting the baseboard heater 10 is schematically represented by arrows 22e, 22f, 22g. The air in the room is heated by natural convection. Temperature distributions for the heated air exiting the baseboard heater 10 based on computer modelling (i.e., computational fluid dynamics) are shown in FIG. 1, by regions identified as H1, H2, and H3. The region identified by reference H1 is the hottest region of air. H2 refers to a region at a temperature lower than H1, and H3 refers to a region at a temperature lower than H2. H1, H2, and H3 are represented in FIG. 1 as being defined by isotherms (temperature gradients) respectively, and those skilled in the art will appreciate that in practice such gradients are not fixed in position, but instead vary over time while the conventional heater is operating. For convenience, the isotherms defining the regions are identified as I1-I5 in FIG. 1.
As is well known in the art, the prior art heater 10 shown in FIG. 1 includes a housing 24 defining a cavity 26 in which the heating element 14 and the fins 12 are positioned. Included in the housing 24 are an inner part 28 attachable to the wall 18, and an outer part 30, the inner and outer parts 28, 30 at least partially defining the cavity 26. In one common arrangement, the inner and outer parts 28, 30 also define an upper opening 32 through which the column of heated air exits the baseboard heater 10, and they also define a lower opening 34 through which ambient air enters the baseboard heater 10. It will be understood that, although a grate is typically positioned in the upper opening, the grate has been deliberately omitted from FIG. 1 for clarity of illustration. Typically, ribs (not shown in FIGS. 1 and 2) are positioned at intervals along the length of the baseboard heater to be support elements, e.g., to support a front panel of the heater housing.
As can be seen in FIG. 1, each fin 12 typically is relatively thin and has a generally uniform shape, with substantially flat vertical sides 36, 38 and a substantially straight top side 40 which is substantially orthogonal to the sides 36, 38. The fin 12 also preferably includes a bottom side 41, which is also generally orthogonal to the sides 36, 38. As is well known in the art, the baseboard heater 10 is attached to the wall 18 so that a sufficient distance “L1” is provided between the bottom edge 41 and a floor 19 to permit an adequate flow of ambient air from the room into the heater 10 at the bottom edges 41 of the fins 12.
As indicated in FIG. 1, when moving through the heater 10, the column of rising air 44 is generally contained between an inner surface 29 of the inner part 28 of the housing 24, and an interior surface 31 of the outer part 30.
In another type of conventional baseboard heater 110, a “beak” 142 is included in the housing 124 (FIG. 2). The beak 142 apparently is intended to guide a column of heated air 144 rising from the heater away from the wall and generally toward the center of the room, in order to heat the room “R” more efficiently. The beak 142 is intended to address a concern that the wide upper opening 32 of the conventional baseboard heater 10 (FIG. 1) allows a significant portion of heat from the warmed air to heat the wall, rather than heating the air in the room.
As shown in FIG. 2, the heat transfer fin 112 is generally similar to the fin 12, with a substantially rectangular shape, having substantially flat sides 136, 138, and a substantially flat top side 140 which is orthogonal (or substantially orthogonal) to the sides 136, 138, and a bottom side 141 which is also substantially orthogonal to the sides 136, 138.
The air flow patterns resulting from operation of the baseboard heater 110 (as determined using computational fluid dynamics) are schematically illustrated in FIG. 2. As can be seen in FIG. 2, ambient air is drawn into the baseboard heater 110 when it is operating (schematically represented by arrows 122a, 122b, 122c, 122d). The incoming air schematically represented by arrows 122a-122d is drawn generally upwardly into the conventional heater 110 when it is operating, to form the column 144 of generally upwardly-moving air (FIG. 2). When the heater is operating, the column of air rises and exits the baseboard heater 120 from an upper region thereof (schematically represented by arrows 122e, 122f, 122g, 122h). Temperature distributions for the column of air 144 (as determined using computational fluid dynamics) are shown in FIG. 2, the column of heated air 144 rising from the heater being divided into regions J1-J3 (defined by temperature gradients I6-I9) of substantially similar temperature. Those skilled in the art will appreciate that the positions of the temperature gradients shown in FIG. 2 are exemplary only, and that in practice the gradients vary over time when the heater 110 is operating.
Based on the computer modelling (i.e., computational fluid dynamics), it appears that the beak 142 tends to result in a “drag” effect (i.e., the Coanda effect) whereby the heated air is guided so that it is directed almost orthogonally to the wall (see, e.g., arrows 122e, 122f, 122g, and 122h).
As is well known in the art, “streaking” (or “staining”) often appears on the wall 18 above the baseboard heater 10, after the conventional baseboard heater 10 has been used for a period of time. The phenomenon of streaking does not appear to have been well understood in the prior art. For instance, in U.S. Pat. No. 5,197,111 (Mills, II et al.), it is stated that streaking is due to dust particles that are charred as they pass by the sheathed element (i.e., the heating element) and are carried upwardly by the warmed air (col. 1, lines 40-44). This suggests that the flow of air past the sheathed element and the heat transfer fins leads directly to streaking. According to this understanding of streaking, therefore, the streaking should appear on the wall in the regions between the ribs. However, this does not appear to be the case.
The shaded regions 20 in FIG. 3 represent typical streaking on the wall 18. As can be seen in FIG. 3, streaking typically occurs in regions of the wall 18 generally above ribs 16, rather than between the ribs. This is contrary to the understanding of streaking outlined in Mills, II et al., referred to above.
Also, it has been determined that the regions 20 of the wall 18 above the conventional baseboard heater 10 where streaking occurs are substantially warmer than the rest of the wall, although the regions 20 are substantially above the ribs 26. Temperature gradients (i.e., isotherms) are shown schematically in FIG. 3 which were determined by taking photographs of the wall above a typical prior art baseboard heater using an infrared camera. In short, it appears from FIG. 3 that the ribs 16 affect the flow of heated air upwardly from the conventional heater to make the parts 20 of the wall where streaking occurs warmer than the rest of the wall.
Referring to FIG. 3, the area within the outer temperature gradient “T1” is warmer than the areas outside it. As can be seen in FIG. 3, the area of streaking 20 on the wall 18 is substantially coincident with the temperature gradient T1. A second temperature gradient “T2” is also shown in FIG. 3, and the areas encircled by this temperature gradient are substantially above the ribs 16. The temperature gradient T2 represents a temperature substantially higher than that represented by T1. As can be seen in FIG. 3, therefore, the parts of the wall where streaking occurs are significantly warmer than the other parts of the wall.
Surprisingly, therefore, the warmest parts of the wall above the conventional baseboard heater 10 are the regions 20 immediately above the ribs. This is surprising because, in the prior art (e.g., Mills, II et al.), it had been assumed that the parts of the wall immediately above the ribs would be cooler.
The reasons for this are not clear. It is believed that the ribs disrupt the upward flow of warmed air exiting from between the fins (i.e., possibly due to the Coanda effect), causing turbulence in the upwardly flowing warmed air above the ribs which results in the streaking. Due to the turbulence, the heated air is directed at least partially towards the wall above the ribs. As a result, tiny particles of dust and dirt in the heated air impinge against the wall generally above the ribs 16. Some of these particles adhere to the wall. Over time, these particles accumulate on the wall in the areas 20 above the ribs 16, to result in streaking (i.e., staining).
Based on the foregoing, it appears likely that some turbulence may also develop in the regions between the ribs at the wall above the heater. In short, although there is much uncertainty about the mechanism or mechanisms that create the streaking, it appears that streaking occurs because the ribs disrupt the upward flow of warm air sufficiently that more turbulence is created at the wall above the ribs than in the intervening regions above the heater. As noted above, the addition of a “beak” to the basic prior art design appears to result in even more turbulence at the wall, not less.